Conventional air conditioning systems cool air in confined spaces by using four main components, including a compressor, condenser, metering device, and an evaporator. These components also provide the basis for most refrigeration cycles. However, as systems become more technologically advanced, additional components are added. Generally, the compressor compresses refrigerant gas to a high pressure, high temperature, superheated gaseous state for use by the condenser. The condenser, in cooling the superheated gas, produces a sub-cooled liquid refrigerant with a high pressure and lower temperature. The metering device, such as an expansion valve, produces a low temperature, low pressure saturated liquid-vapor mixture from the sub-cooled liquid. Finally, the evaporator converts the saturated liquid-vapor mixture, to a low temperature, low pressure superheated gas during air cooling for use by the compressor. The overall performance and efficiency of refrigeration cycles are directly dependent upon the heat transfer provided by the condenser, evaporator, and compressor oil cooler, The overall performance is further dependent upon the performance and lubrication of the compressor.
During operation, most compressors use lubricants which reduce wear and/or seal gaps in the compressor to prevent internal refrigerant leakage. By maintaining the compressor lubricants at relatively low temperature, compressor efficiency and reliability are increased, providing improved lubricant sealing properties due to increased oil viscosity, improved compressor cooling, and decreased frictional wear. For example, screw type compressors utilize counter-rotating rotors to compress refrigerant gas. Such compressors rely on lubricants to reduce friction between mating parts and seal gaps between the rotors and crankcase thereof. Typically, the refrigerant includes some amount of the acquired lubricants before entering the compressor, but some rotating compressor technology injects the oil into the compression process separately.
More particularly, refrigerant enters a compressor in vapor form and is compressed, thereby increasing in pressure and temperature. The compressor releases the refrigerant and lubricant mixture and the mixture subsequently travels throughout the refrigeration system via a series of closed conduits. In some refrigeration cycles, the refrigerant and lubricant mixture exits the compressor and enters an oil separator. The oil is separated from the refrigerant and the refrigerant is routed to a condenser where the heat removal operation via a cooling medium such as outdoor air, occurs on the refrigerant. With heat removed, the refrigerant exits the condenser at high pressure and lower temperature. In a liquid form, the compressor lubricant flows through an oil cooler, such as a heat exchange apparatus, similar to the condenser, wherein air is the cooling medium. The cooled oil flows back to the compressor, functioning to lower the refrigerant discharge temperature and increase the efficiency of the compressor. The refrigerant flows from the condenser to the metering device, such as an expansion valve, wherein temperature and pressure of the refrigerant are reduced for subsequent use by the evaporator and results in cooling of the air of the desired space. Between the condenser and the evaporator, refrigeration cycles such as this may also include an economizer circuit for use in further cooling of the main refrigerant stream. In such cases, an economizer heat exchanger is provided through which the main refrigerant stream passes for cooling. A secondary refrigerant flow off-shooting from the main line exiting the condenser is passed through an auxiliary metering device for achieving intermediate pressure and temperature refrigerant. This refrigerant is used in further sub-cooling of the main refrigerant flow prior to its passage through the metering device. With the main refrigerant stream cooled in this manner, it can be used in another heat exchange mechanism for further lowering its temperature by the expense of the refrigerant traveling from the evaporator to the suction port of the compressor.
As indicated above, typically oil is cooled by using a separate oil cooler. However, the prior art does include refrigeration systems which combine the oil cooling with other cooling steps in a simultaneous process. For example, U.S. Pat. No. 5,570,583 discloses the integration of an oil cooler with a refrigerant condenser. The system uses the refrigerant to cool the compressor lubricant. However, a parasitic loss of compressor capacity occurs because the main refrigerant stream is used to directly cool the oil and in the process, evaporates a certain amount of refrigerant, reducing available sub-cooling. Accordingly, some amount of the compressor power is increased and the system capacity is decreased. The use of separate oil coolers, in the form of heat exchangers as described above, substantially adds to the part count of refrigeration systems, as well as requiring the use of additional refrigeration circuits or additional external energy source to accomplish cooling. At the same time, by combining two heat transfer processes in one heat exchanger they can be arranged in the most efficient optimal manner through heat flux redistribution, which is not possible otherwise.
There exists a need, therefore, for an improved refrigeration cycle including a more efficient design for cooling the compressor lubricant.